A Case Study of Atmospheric Conditions at 4-19 km over Vandenberg Air Force Base during Passage of a Cyclone.

Continuous observations from a 50-MHz radar of the vertical profiles of winds, refractivity turbulence structure constant (C²_n), turbulent kinetic energy density (²_t) up to two orders of magnitude), and hourly standard deviation of the vertical velocity (_w) are combined with routine rawinsonde ascents and weather observations to study changes in turbulence intensity over Vandenberg Air Force Base, California, during 21-27 July 2001. Early and late in this period the flow aloft was relatively steady and turbulence intensities in the upper troposphere were very low. For about 36 h around 25 July, the flow at midlevels (about 600-150 hPa) was disturbed by the passage of a cutoff cyclonic circulation center. During the low's passage, C²_n and ²_t intensities were enhanced by up to about 15 dB. The enhancements of C²_n and ²_t are highly correlated with each other, as well as with the vertical shear of the horizontal wind and synoptic-scale relative vorticity. The increased turbulence intensities apparently are due to in situ processes, such as shear instabilities associated with the low. Brief episodes of enhanced _w, taken as an indicator of enhanced gravity wave activity, are also associated with the low's passage, and they are also apparently due to an in situ process, such as shear instability or geostrophic adjustment. In the lower stratosphere, above 16 km, winds show a periodic oscillation, with a period near the local inertial period and with downward phase progression. There are bursts of enhanced C²_n near the lower edge of this oscillation. The oscillation does not appear to be affected by the low's passage.